The present invention is drawn to a process and apparatus for purifying a contaminated gas stream containing contaminants by the conversion of the contaminants into less harmful products.
In spite of decades of effort, a significant need remains for an advanced technology to control stationary source emissions of volatile organic compounds (VOCs) as for example benzene, chlorinated volatile organic compounds (CVOCs) as for example trichloroethylene, and toxic air pollutants (TAPs) as for example acrylonitrile. A particular need exists for technology which controls emissions from industrial processes and other applications where low concentrations of VOCs and TAPs are present in high flow rate air streams.
Dilute air stream pollution control is becoming recognized as a major environmental control issue for the United States industrial community at large. For example, the control of indoor air pollution associated with solvent degreasing operations is necessary, including the dilute emissions associated with exhaust ventilation fans. Also, air stripping of contaminated groundwater produces dilute air emissions for which current technology provides no satisfactory solution. Catalytic combustors are available, but require processing tremendous volumes of air and result in uneconomic performance. Thermal incinerators require excessive supplemental fuel for dilute mixtures, and exhibit uncertain selectivity when CVOCs are involved. Gas membrane processes are only now emerging for gas separation, and are ill-suited for dilute mixtures. Pressure swing adsorption using zeolites or resins is not applicable to dilute mixtures, and rotating wheel adsorbers are uneconomic for such dilute concentrations of organics. Packed bed activated carbon adsorption is widely practiced, but creates a hazardous solid waste which is increasingly difficult to manage. Carbon regeneration by steam is costly, and is generally economic only for very large scale operations. Landfill options for spent carbon will become more limited, as it involves transportation and disposal of hazardous wastes, particularly for CVOC applications.
Control of indoor air pollution is also of growing importance, with the objective of enhancing workplace environmental health and safety protection. The Occupational Safety and Health Administration (OSHA) is promulgating new regulations to reduce workplace exposure to indoor air contaminants such as CVOCs. Many CVOCs are particularly toxic. Certain CVOCs are suspected carcinogens, others are linked to possible birth defects, and still others are suspected active precursors in the destruction of the stratospheric ozone layer. Of the 189 targeted air toxics in the Clean Air Act Amendments of 1990, about one-third of the compounds are chlorinated. By the standards of conventional air pollution control technology, indoor air pollution is at exceedingly dilute concentrations. An effective and economic pollution control technology in dilute air systems is the objective of this invention.
In spite of considerable efforts of researchers in the field, most UV photolytic and photocatalytic systems exhibit shortcomings in performance which limit their commercial utility for air pollution control. One shortcoming of note is the propensity of such processes to produce undesirable byproducts of incomplete oxidation, both by the photolytic treatment of contaminated air and by the photocatalytic treatment of contaminated air. Indeed, some such byproducts can be more harmful than the original contaminant being removed, such as the formation of phosgene byproduct through both photolytic and photocatalytic oxidation of trichloroethylene. As used herein, photolytic oxidative destruction is defined as the reaction of contaminants in an oxygen-containing gas stream as a result of the action of the ultraviolet radiation, by oxidation reactions, decomposition reactions, bond-scission reactions, and the like. Photocatalytic oxidative destruction is defined as the reaction of contaminants from an oxygen-containing gas stream as a result of the action of the ultraviolet radiation on the surface of a photocatalyst, by oxidation reactions, decomposition reactions, bond-scission reactions, and the like.
Photolysis could appear to be an efficient and effective means to control emissions of air-borne contaminants. It has been known for decades that contaminants in an air stream can be treated by passing the air through vessels simply containing UV lamps. Legan, in U.S. Pat. No. 4,045,316 has disclosed such a process, augmented by ozone addition. Knoevenagel and Himmelreich, in U.S. Pat. No. 3,977,952 have disclosed such a process without the augmentation by ozone. Such methods would appear to be simple and cost-effective, but they have not been used commercially. Their primary deficiency is that the methods also produce partially oxygenated byproducts, and there has been no way to control this undesirable byproduct formation. Such byproducts were recognized Kitchens, in U.S. Pat. No. 4,144,152. More recent studies have confirmed the byproduct formation with more sophisticated analytical techniques (see Haag, W. R. and M. D. Johnson, "Direct Photolysis of Trichloroethene in Air: Effect of Co-contaminants, Toxicity of Products, and Hydrothermal Treatment of Products", Environmental Science and Technology, 30, No. 2, 414-421, 1996; Spaeder, T. A., "Experimental Studies of an Ethanol-Air Flow Subjected to UV Light", Air & Waste Management Association, 87th Annual Meeting, Cincinnati, Ohio, 1994; and Bolton, J. R. et al., "Homogeneous Photodegradation of Pollutants in Air", Air & Waste Management Association, 87th Annual Meeting, Cincinnati, Ohio, 1994).
Photocatalysis using UV radiation has also been tested for decades as a means to destroy air pollutants. Juillet, et al., in U.S. Pat. No. 3,781,194 disclosed the use of titania for the oxidation of gaseous hydrocarbons, and found primarily aldehydes and ketones in the reaction products. More recently, extensive amounts of byproducts from the photocatalytic oxidation of air-borne contaminants have been found by current analytical techniques (see Nimlos, M. R., et al., "Direct Mass Spectrometric Studies of the Destruction of Hazardous Wastes, 2. Gas-Phase Photocatalytic Oxidation of Trichloroethylene of TiO.sub.2 : Products and Mechanisms", Environmental Sciences and Technology, 27, No. 4, 731-740, 1993; Hall, R. J., et al., "Computational and Experimental Studies of UV/Titania Photocatalytic Oxidation of VOCs in Honeycomb Monoliths:, Third International Conference on TiO.sub.2 Photocatalytic Purification and Treatment of Water and Air, Orlando, Fla., Sep. 23-26, 1997; Obee, T. N. and S. O. Hay, "The Augmentation of L TV Photocatalytic Oxidation with Trace Quantities of Ozone", Third International Conference on TiO.sub.2 Photocatalytic Purification and Treatment of Water and Air, Orlando, Fla., Sep. 23-26, 1997).
Naturally, it would be highly desirable to provide a process and apparatus for purifying a contaminated gas stream containing contaminants by the conversion of the contaminants into less harmful byproducts while avoiding the shortcomings associated with known processes and apparatus described above.
Surprisingly, it is found that the combination of a photolytic step with a photocatalytic step as taught the present invention, each of which produce undesirable byproducts individually, results in a more efficient process which eliminates or reduces the production of said undesirable byproducts from the combined stages. Without being bound by theory, it appears that the pretreatment of gas-phase contaminants in said photolytic stage efficiently produces partially oxygenated species which, in turn, are much more efficiently converted in the photocatalytic stage to less harmful products. Because the photolytic stage produces byproducts which are particularly easy to destroy in the photocatalytic stage, the combined two-stage process of the present invention produces little or no byproducts.
Accordingly, it is the principle object of the present invention to provide a process and apparatus for purifying a contaminated gas stream by conversion of the contaminants into less harmful products.
It is a further object of the present invention to provide a process and apparatus as mentioned above which combines photolytic and photocatalytic stages wherein the process results in a synergistic effect.